A Near Field Communication (NFC) system comprises a reader and one or more targets, which may alternatively be referred to as tags, cards, NFC-enabled phones, etc.
The reader and target communicate through inductive coupling of their respective antennas using, for example, Amplitude-Shift Keying (ASK) modulation. The reader transmits an unmodulated carrier signal C(t) generally at a 13.56 MHz carrier frequency (Fc). A target receiving this carrier signal C(t) may communicate data to the reader by modulating the carrier signal with the data to generate a load modulated signal M(t). In the case of passive load modulation the target modulates the data on the carrier signal by switching between two impedance loads. In the case of active modulation, the target generates a carrier synchronized to the carrier from the reader. The amplitude and phase of the resultant modulated carrier signal CM(t) is a sum of the unmodulated carrier signal C(t) and the load modulated signal M(t).
There impedance variations translate into a carrier amplitude modulation that the reader detects. There is also a parasitic carrier phase difference between unmodulated and modulated states.
Further, there is typically an operating volume within which the communication between the reader and target is successful. However, there are typically one or more points in space within this operating volume where the load modulation amplitude is zero or near zero and not sufficient for the reader to demodulate. Such points are known as “communication holes”. For example, if there is an operating volume of about 4 cm, somewhere within the 4 cm operating volume, for example, at around 2 cm, the amplitude modulation caused by the load modulation may be close to zero. This means that both below and above 2 cm there is load modulation amplitude that can be decoded, but at 2 cm the load modulation amplitude is zero.